The present invention relates to capacitors for use in electrical circuits and the like, and more particularly, to a variable capacitor that is adapted for fabrication using micromachining.
Parallel plate capacitors having a capacitance that depends on the potential difference between the plates are well known in the electrical arts. The devices typically consist of a fixed bottom plate and a movable top plate that is held over the bottom plate with the aid of a number of springs. As the potential difference between the plates is increased, the top plate is pulled toward the bottom plate by the electrostatic forces against the restoring forces provided by the spring system. Variable capacitors of this type are described in Darrin J. Young and Bernhard E. Boser, xe2x80x9cA Micromachined Variable Capacitor for Monolithic Low-Noise VCOs,xe2x80x9d Technical Digest of Solid-State Sensor and Actuator Workshop, 1996, pp. 86-89 and in Aleksander Dec and Ken Suyama, xe2x80x9cMicromachined Electro-Mechanically Tunable Capacitors and Their Applications to RF IC""s,xe2x80x9d IEEE Transactions on Microwave Theory and Techniques, vol. 46, no. 12, Part 2, December 1998, pp. 2587-2596.
The usefulness of such variable capacitors in mass-produced electronics depends on the reproducibility of the capacitors, particularly the voltage versus capacitance function provided by each capacitor. In particular, the spring forces provided by the suspension springs must be repeatable from device to device. This, in turn, requires that the stress in the springs in the absence of an applied voltage be repeatable and, for a given voltage, be repeatable during operation. Here, it should be noted that a tensile or compressive stress on the springs makes the effective spring constant higher or lower, respectively, and hence, alters the capacitance as a function of voltage curve.
The top plate is typically constructed from a conducting film that is deposited using conventional integrated circuit fabrication techniques. Stress in this film can translate into stress in the springs if the film cannot change size to relieve the stress. As a practical matter, stress in the film or films that form the top capacitor plate is very difficult to control for a number of reasons. First, during fabrication, there is normally residual stress in the film or films that make up the top plate and spring. Second, during packaging, the die-attachment material applies stress to the bottom of the substrate.
Finally, during operation, the temperature varies. The substrate (e.g., silicon or glass) and the package usually do not have the same thermal expansion rate as the films that make up the top plate. Hence, a change in temperature results in a variable stress in the plate-spring structure. Consider a prior art capacitor such as those discussed in the above-cited references in which the top plate and springs are made of metal with a relatively large thermal coefficient of expansion (TCE), and the substrate is made of silicon or glass with a relatively low TCE. If a voltage has been applied to the capacitor, as the temperature falls the plate and spring shrink faster than the substrate, increasing the tension, pulling the plates apart, decreasing the capacitance.
In principle, the problems caused by temperature fluctuations can be reduced by using the same material for the top plate as is used for the underlying substrate, and hence, reduce the mismatch in TCE between the two structures. However, for micromachined devices, this practically limits the material to silicon, as other materials have not historically been used for both the substrate and plate/spring. Unfortunately, silicon has a much higher resistivity than metals, and hence, such capacitors are poorly suited for capacitors in high-frequency applications.
Broadly, it is the object of the present invention to provide an improved micromachinable variable capacitor.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a capacitor having a bottom plate, a top plate and a support connected to the center portion of the top plate for positioning the top plate over the bottom plate and separated therefrom by a gap. The outer portion of the top plate moves relative to the bottom plate when a potential is applied between the top and bottom plates. The outer portion of the top plate may be connected to the central portion of the top plate by a plurality of springs such that the movement of the top plate relative to the bottom plate is accommodated by bending at least one of the springs. The capacitor may also include an insulating layer between the top and bottom plates disposed so as to prevent the top plate from shorting to the bottom plate. In addition, a spacer for setting the minimum distance between the outer portion of the top plate and the bottom plate may also be included. In one embodiment of the invention, the springs are shaped to relieve thermal stress between the outer portion of the top plate and the center portion of the top plate.